This invention lies in the field of seismic exploration systems. More particularly, it lies in the field of seismic recording systems, in which the recording amplifiers are positioned at the geophone locations and their outputs are either transmitted by common conductors to the recording trucks or recorded on a tape recorder at the geophone location.
In the prior art, seismic recording systems have traditionally involved setting out a plurality of geophone groups at spaced locations and connecting each of these geophone groups by separate conductor pairs to separate amplifier channels in a recording truck at some distance from the geophone locations. (See Milton Dobrin, "Introduction to Geophysical Prospecting". McGraw-Hill, 1952, pp. 201-208.) In the recording truck, a complex amplifier system was provided in which the gain could be ranged (automatically changed) over a wide range. Such a system is described in U.S. Pat. No. 3,315,223. This gain-changing was done in steps differing by a factor of 2.sup.n (where n is an integer) at a high rate of speed.
With the advent of recording systems requiring many more channels, up to 96 to 200 or more, it has become convenient to provide simpler amplifiers at each of the geophone locations or more usually to serve a small group of geophone locations. One such system is described by U.S. Pat. No. 3,806,864, in which two seismic recording channels are provided at the location of two adjacent geophone groups and the outputs of the amplifiers are locally recorded on magnetic tape cassettes. Because of the difficulty (in size, weight, and power drain) of having very complex high-speed, gain-ranging amplifiers, these newer amplifiers are of lesser total gain range. However, there are adjustable or present types of preamplifiers that can be set manually to provide adjustable gain in addition to the gain range set by the amplifier itself.